1. Field of Invention
The present invention relates generally to the field of inorganic fiber production and systems, and more specifically to heat exchange and control strategies useful in flame attenuation fiberization processes producing inorganic microfibers and other fibers.
2. Related Art
One process for manufacturing fine diameter mineral fibers, e.g. discrete length, glass fibers typically ranging from about 0.2 microns to about 7.0 microns in diameter, is the flame attenuation process. In this process, an electrically or gas flame heated pot or melter containing a molten fiberizable material such as glass batch materials or preformed marbles are melted and drawn from a plurality of outlet orifices of a bushing to form continuous primary filaments. The primary continuous filaments are drawn from the heated pot or melter by pull rolls which also function to feed the continuous primary filaments into a high temperature, high energy, gas flame that further attenuates the continuous primary filaments and forms short length, fine diameter fibers from the continuous primary filaments. These attenuating burners have extremely high gas flow rates in order to stretch the filaments while they are heated so as to reduce their diameter. As the attenuated filaments cool below the melting temperature of the glass, these filaments are broken by the force of the attenuating blast into fibers within a predetermined range of lengths, this range being a function of the operational parameters and the configuration of the attenuation zone. A filament guide with a plurality of grooves therein, guides and supports the continuous primary filaments into the flame so that the continuous primary filaments can be introduced into the flame at a specific location without being blown haphazardly about by the flame. The discrete length, fine diameter fibers, thus formed, are generally collected to form a fibrous blanket with the fibers randomly oriented within the blanket.
Energy costs continue to increase, spurring efforts to find ways to reduce the amount of fuel in mineral fiber manufacturing. The high velocity attenuation blast entrains cooler air from its surroundings. This low energy, low velocity air is mixed with the attenuation stream thereby diluting it and reducing both its temperature and velocity. The capability of the attenuating apparatus to reduce fiber diameter (i.e., to improve the filtration or insulating capabilities of the material) is hampered by this unrestricted stream dilution. To offset the disadvantages of dilution, more gas must be burned to produce higher temperatures. Fiberization process operators have resorted to restricting dilution by providing a shroud around the attenuation region which limits the entrainment of dilution air by restricting the access of the surroundings to the region. The shroud also confines the heat thereby increasing the temperature in the attenuation zone. Several openings are provided in the shroud to permit a restricted amount of dilution air to be beneficially entrained by the attenuation stream. The dilution air from at least one of the openings may be provided with a preheater which uses waste heat rising from the attenuating burner to heat the air. The position of the stream of gases can also be adjusted within the shroud by adjusting the amount of air inspirated above and below the centerline of the blast or stream. The inspirated air stream may be directed to create turbulence in the combined stream, causing the primary filaments to adopt a serpentine path within the attenuation zone which increases the length of time each primary is exposed to the heat of the attenuation zone and thereby improves fiber attenuation (i.e., reduces fiber diameter).
Despite these advances in the art, there is still a need for further energy efficiency in mineral fiberization processes. Because of the tremendous amounts energy required in glass tank furnaces, steel blast furnaces, and rotary furnaces, combined with regulations limiting the amount of NOx and SOx emissions, operators in those industries have used oxygen-enriched air to decrease energy usage and emissions. These tend to be very high temperature processes (at least, 820° C., 1500° F.). In very high temperature processes in large furnaces, NOx formation is promoted by long residence times of oxygen and nitrogen molecules in hot regions of the flame and the furnace. The use of substantially pure oxygen (about 90% O2 or higher) instead of air as the oxidant has proven to be very successful in reducing the NOx emissions by as much as 90%, since all nitrogen is eliminated. However; substitution of air by substantially pure oxygen increases the flame temperature, and thus creates regions in the larger furnaces where the reactivity of nitrogen with oxygen is high, and wherein the formation of NOx may proportionally increase, even though it is globally decreased when compared to combustion with air. Use has been made of regenerative and recuperative furnaces in the aforementioned industries to recover some of the heat in the high temperature effluent gases. Regenerative glass tank furnaces use hot combustion gases that otherwise would be vented to the atmosphere to heat an intermediate heat transfer material, such as ceramic balls held in towers. Typically two towers are used, so that one tower is heated by combustion gases while the other tower has air flowing there through to preheat the combustion air before entering burners. The towers are switched in cyclic fashion. Recuperative glass tank furnaces preheat combustion air using heat exchange between cool air and combustion gases. In addition to air preheating, commercial grade oxygen and oxygen-enriched air may be preheated employing direct or indirect heat exchange (through one or more heat exchange fluids, such as an inert gas) using specially designed heat exchangers. However, none of these techniques, despite their being available for sometime, have ever been used in mineral fiberization processes and systems. This may be due to any of a variety of factors. Not only are the fields of use quite different, but the nature of the molten material and equipment being different (fibers vs. large pools of molten material, usage of burners to attenuate fibers vs. usage of burners for melting) leads to very different problems to be solved, despite the fact that decreased energy usage is a common goal of many industries, including both the float glass and mineral fiber industries. As the end use of mineral fibers depends on the physical properties of the fibers, such as their ability to be dispersed in liquids and slurries, or their ability to function as filter media or insulation, producers are careful not to change a process that produces acceptable fibers for a small decrease in energy consumption.
Because of this it would be an advance in the fiberization art to reduce energy requirements a significant amount in mineral fiberization processes to make their implementation attractive, particularly in situations where the fiber physical properties are acceptable, or even better than acceptable, in terms of higher quality fibers and products employing the fibers, such as filtration and insulation products.